1. Field of the Invention
The present invention relates to a circuit board and a structure used for electronic devices (e.g., audiovisual devices, electrical appliances, communication devices, computer devices, and the peripheral devices thereof).
2. Description of the Related Art
A structure obtained by mounting or embedding an electronic component on or in a circuit board has been used in electronic devices.
As shown in FIG. 9, a circuit board 9 includes a plurality of conductive layers 91 apart from each other in the thickness direction, insulating layers 90 interposed therebetween, and a plurality of via conductors 92 electrically connecting the conductive layers 91. The via conductors 92 are formed in the insulating layers 90 to be apart from each other in a plan view. In the circuit board 9, the conductive layers 91 and the via conductors 92 constitute a power supply line and a signal line (refer to Japanese Unexamined Patent Application Publication No. 8-116174).
Focusing on a single conductive layer 91 (other than a conductive layer 91′ that is located in an outer layer), each of the via conductors 92 is connected to an upper surface 93 and a lower surface 94 of the conductive layer 91. For example, a current flows in the order of the via conductor 92 (thickness direction), the conductive layer 91 (plane direction), and the via conductor 92 (thickness direction) as indicated by an arrow in the drawing. Since a current flows through a path having the shortest distance in a conductive region, the current is concentrated in a portion 95 (a portion circled in the drawing) in the connected portion between the via conductor 92 and the conductive layer 91, which causes unbalanced current density. Therefore, in the portion 95, metal atoms contained in the via conductor 92 may migrate due to a collision between electrons and metal atoms. This phenomenon is called electromigration. Electromigration decreases the metal density in the portion 95 (portion in which a current is concentrated) of the connected portion between the via conductor 92 and the conductive layer 91. Consequently, cracking occurs in the connected portion between the via conductor 92 and the conductive layer 91, which sometimes decreases the reliability of the circuit board 9.
Electromigration mainly occurs in a power supply line, but it is highly likely to occur in a signal line too if the size of a signal line becomes smaller in the future.
Focusing on a conductive layer 91′ that is located in an outer layer, a single via conductor 92′ is connected to the conductive layer 91′. Even in this case, electromigration occurs in a portion 95′ of the connected portion between the conductive layer 91 and the via conductor 92′, which sometimes decreases the connection reliability of an electronic component 96 to the circuit board 9.